Manholes are used to access various underground spaces. For example, underground electrical distribution lines are sometimes formed by splicing segments of insulated electrically conductive lines together. It is conventional to leave at least the joint formed by splicing multiple segments together in a relatively exposed condition in an underground space and to install a manhole so maintenance or repair workers can access the underground space to do maintenance or repair work. Faults in the electrical distribution line in the underground space can generate combustible gas, which can sometimes be ignited by sparks from the faulty electrical distribution line or other ignition sources. In the event combustible gas accumulates in the underground space and is ignited, an explosion may result. One of the dangers presented by any such explosion is that it could blow the manhole cover into the air. Because manhole covers typically weigh several hundred pounds, there is potential for injury or death to people and/or significant property damage from the manhole cover in the event of an explosion.
U.S. Pat. No. 7,712,995 (Stadler) discloses a method of controllably venting gases generated by an explosion in a manhole space. Stadler's solution is to use a specially formed manhole cover having a lug 24 and lock 20 formed on the underside of the manhole. The lug and lock are positioned on opposite sides of the manhole cover at its perimeter. A skirt 66 extends down from the bottom of the manhole cover and has a plurality of openings 68 spaced circumferentially around the perimeter of the manhole cover. In the event of an explosion, the manhole cover is lifted off the frame of the manhole to allow gas to be vented between the manhole cover and frame. In the first stage of an explosion, the manhole cover is lifted by the gas pressure until the lug and lock catch on an annular shoulder 18 extending inward from the manhole frame. If the venting provided at this first stage is insufficient, the pressure from the gas breaks a shear pin 32 in the lock which allows the side of the manhole cover on which the lock is positioned to rise further off the frame to allow additional venting.
In order to access the space covered by Stadler's manhole cover, workers use a tool to retract a latching member 26 of the lock. Even with the latching member retracted, the lug 24 prevents the manhole cover from being lifted straight off the frame. However, workers are able to remove the unlocked manhole cover and enter the space below by lifting the side of the manhole on which the lock is positioned. The lug prevents the other side of the manhole from being lifted more than a short distance off the frame, but once the side of the manhole cover on which the lock is positioned is lifted high enough, the lug can be disengaged from the shoulder and the cover can be removed by pulling the manhole cover away from the portion of the shoulder engaged by the lug.
The present inventor has discovered improved systems and methods for venting gas from an underground space covered by a manhole cover, which will be described in detail below.